The invention relates to a microchip laser comprising a monolithic resonator having a birefringent laser crystal, wherein a laser beam which has a laser wavelength and which is decoupled from the resonator emerges from the resonator along a laser beam axis and the length of the resonator in relation to the direction of the laser beam axis is shorter than 150 μm.
Microchip lasers are solid-state lasers comprising a monolithic resonator and distinguished by a particularly small construction. End mirrors of the resonator are formed by coating the active laser medium and/or an optical element having or forming an end mirror, for example a SESAM (Semiconductor saturable absorber mirror), is cohesively connected to the active laser medium. Such connection techniques are known as “bonding”.
Short pulses can easily be formed by microchip lasers due to the short resonator lengths. Hence, pulses with pulse lengths of less than one nanosecond or else less than 100 picoseconds, in extreme cases of less than 20 picoseconds, may be achieved by means of Q-switching. Such pulses are of interest, for example, for micro material processing.
Existing mode-coupled lasers, by means of which such short pulses may be generated, have a substantially larger embodiment. Typical resonator lengths are more than 1 m and such resonators may be housed in a cube with an edge length of more than 10 cm as a result of multiple folding. By contrast, a microchip laser resonator may be housed in a cube with an edge length of less than 1 mm.
Gain switching may also be carried out for a microchip laser, wherein pulses with pulse durations in the nanosecond range, or else shorter, may be generated. In principle, a microchip laser may also be operated continuously (=in the cw mode).
Microchip lasers have a potentially cost-effective production because a planar laser structure may be produced in a batch process (wafer process) such that more than 100 laser resonators may by all means be obtained from an area of 10 mm×10 mm.
A particular stability of a microchip laser is facilitated by the monolithic structure.
Microchip lasers of the type set forth at the outset with particularly short pulse durations emerge from WO 2011/147799 A1 and from Mehner, E., et al., “Sub-20-ps pulses from a passively Q-switched a laser chip at 1 MHz repetition rate”, OPTICS LETTERS, volume 39, number 10, May 15, 2014, 2940-2943. These publications specify further documents in which microchip lasers in which pulses with pulse lengths under 100 ps are achieved by Q-switching are disclosed.
Microchip lasers usually emit at a single frequency in the case of short resonator lengths and low gain bandwidths of the active laser material, i.e. the laser beam decoupled from the resonator has a defined laser wavelength. The short resonator length leads to a large “free spectral range”, i.e. a large distance between adjacent wavelengths which are resonant in the resonator.
A laser crystal formed by a vanadate, in particular Nd3+:YVO4, is often used as active laser medium for microchip lasers. As laser material, Nd3+:YVO4 has advantageous properties such as a comparatively high small signal gain and good absorption of the pumping radiation. Nd3+:YVO4 is a birefringent crystal, with previous experience having shown that a microchip laser comprising Nd3+:YVO4 as a laser crystal may laser with a polarization corresponding to the ordinary ray or corresponding to the extraordinary ray.
Birefringent crystals are optically anisotropic, in the direction of the crystal optical axis in the case of optically uniaxial birefringent crystals with an exception for a light incidence. In the case of optical uniaxial birefringent crystals, the refractive index is independent of the polarization direction of the light in the case of an incidence parallel to the only crystal optical axis. For an incidence of light at an angle to the crystal optical axis, the light beam is divided into two linearly polarized sub-beams, the polarization directions of which are at right angles to one another and which are referred to as ordinary ray and extraordinary ray. The refractive indices differ for the ordinary ray and extraordinary ray, with the difference between the refractive indices assuming the maximum value thereof in the case of a direction of incidence of light at right angles to the crystal optical axis. In the case of such an incidence of light at right angles to the crystal optical axis and onto an entrance surface of the birefringent crystal at right angles to the direction of incidence of light, there is no spatial separation here between the ordinary and extraordinary rays polarized at right angles to one another.
A birefringent crystal may also have more than one crystal optical axis.